Rear side BSF/emitter patterning of SHJ-IBC solar cells using selective deposition and hydrogen plasma etching of a-Si:H

Abstract: Silicon heterojunction interdigitated back-contact (SHJ-IBC) solar cells have attracted considerable attention because of their potential to achieve very highly efficiency. However, the back contacting scheme leads to additional fabrication complexity mainly resulting from the formation of interdigitated n- (back surface field (BSF)) and p-type (emitter) hydrogenated amorphous silicon (aSi:H) strips. Photolithography is widely used for patterning of the interdigitated strips, but this is a costly and impractical technique for industrial applications. This work focuses on the development of simple BSF/emitter patterning approaches of SHJ-IBC cells to replace photolithography and two methods, selective deposition (SD) and dry etching of a-Si:H, have been evaluated. Selective deposition of materials is a promising approach for electronic device fabrication, which allows materials deposition on pre-defined areas while no deposition occurs on other parts of the device. As a result, costly lithography and etching steps can be avoided. Selective deposition of a-Si:H at low temperature (~200C) using plasma-enhanced chemical vapor deposition (PECVD) technique, is a novel approach for rear side emitter patterning of SHJ-IBC solar cells. The first target of this study was to selectively deposit a-Si:H on crystalline silicon (c-Si) while avoiding deposition on the SiOx mask. The second target was to achieve a sufficient quality a-Si:H/c-Si interface passivation using selectively deposited silicon film. The SD of a-Si:H was realized by short deposition of a-Si:H followed by etching using hydrogen plasma. Hydrogen atoms can selectively eliminate strained bonds in a-Si:H films. Proper hydrogen plasma exposure allows to discriminate between Si-Si bonds on different substrates. As such, substrate-SD of a-Si:H is possible. Repetition of short deposition followed by hydrogen plasma etching leads to net deposition on one substrate, i.e., c-Si, but not on mask layers, i.e., SiOx. Two deposition approaches were used to develop SD. The first approach is "Time-modulated power"; where deposition and etching is controlled by radio frequency power. The second deposition approach is "Time-modulated SiH4", where film deposition and etching is controlled by pulsed SiH4 flow. Spectroscopic Ellipsometry (SE) and Transmission Electron Microscopy (TEM) were utilized to measure selectivity, film thickness, and morphology. Effective carrier lifetime (τeff) was measured to check the c-Si surface passivation quality using Quasi-steady-state photo-conductance (QSSPC) method and Photoluminescence (PL) image. From TEM it was observed that, although selectivity is achieved in this work, crystallization of the deposited Si film on c-Si results in poor passivation quality, which is probably induced by long hydrogen plasma exposure. To reduce the crystallization rate, NF3 plasma treatment was carried out on c-Si surface before SD. The suppression of silicon epitaxial growth was observed. This is due to the transformation of c-Si surface bonding configurations which was confirmed by measurement of Attenuated total reflectance Fourier transform infrared spectroscopy. The QSSPC and PL results indicated improved passivation quality using NF3 plasma treatment before SD. The other part of this study concerned the etching of a-Si:H (n+) layer in i/n+ a-Si:H stack using hydrogen plasma followed by in-situ re-deposition of a-Si:H(p+) layer. With the aid of in-situ deposition, the vacuum break of PECVD, HF dip of c-Si wafer prior to a-Si:H(p+) deposition, wafer rising and drying can be skipped. A sufficient c-Si surface passivation is here required after the etching and re-deposition processes. According to the SE results, a stable and uniform etching of a-Si:H(n+) film with etching rate of 1.4±0.1 nm/min was achieved. An excellent surface passivation quality with τeff of above 8ms was obtained after etching of a-Si:H(n+) and re-deposition of a-Si:H(p+) layer. A thicker a-Si:H(i) layer was proven to be beneficial to prevent passivation degradation during hydrogen plasma etching. The preliminary results suggest that this is a promising method to replace currently used etching methods that remove the whole a-Si:H(i/n+) stack, significantly simplifying rear side patterning steps for SHJ -IBC solar cell devices.